The present invention relates to an image recording apparatus such as an electrophotographic copier or printer, and especially to an image recording apparatus in which: a toner image is obtained by developing a latent image on a photoreceptor; the toner image is physically transferred onto a recording sheet by a transferring/conveying belt device having a rotating transferring/conveying belt; and after that the recording sheet, to which the toner image has been adhered, is conveyed to a fixing means for fixing the toner image on the recording sheet.
In the image recording apparatus such as an electrophotographic copier or printer, for example, the peripheral surface of a photosensitive drum, is uniformly charged by a charger, the peripheral surface of the photosensitive drum is exposed, and an electrostatic latent image is formed thereon. The latent image is developed into a toner image by a developing means. The toner image formed on the peripheral surface of the photosensitive drum is transferred onto the recording sheet, which is conveyed in timed relation with the photosensitive drum, by a physical means. The recording sheet, onto which the toner image has been transferred, is separated from the photosensitive drum, conveyed to a fixing means, and delivered to the outside of the apparatus after the toner image has been fixed onto the recording sheet.
The transfer means, by which the toner image is adhered to the peripheral surface of the photosensitive drum an transferred onto a recording sheet, functions as follows: charging, the polarity of which is negative with respect to the charging polarity of the toner, is carried out so that the toner image is transferred onto the recording sheet by a transfer device by which charging is conducted from the back of the recording sheet; and after transfer, an AC high voltage is impressed upon the recording sheet so that the recording sheet is discharged and separated from the photosensitive drum. However, it is difficult to positively maintain transfer properties and separability. Especially, when the diameter of the photosensitive drum is large, it is difficult to separate the recording sheet from the drum. In this case, the recording sheet, onto which the toner image has been transferred, is stuck to the photosensitive drum, and this is apt to cause paper jamming, which is disadvantageous. A transferring belt device is used to overcome the above disadvantage.
The transferring belt device is structured in the manner that the transferring/conveying belt is stretched around a plurality of rollers, and is rotated at the same speed as that of the photosensitive drum. In the transferring belt device, a charge supplying means is provided by which the recording sheet is electrostatically stuck to the transferring/conveying belt. The recording sheet is contacted with the photosensitive drum in the transfer section, while the recording sheet is stuck to the transferring/conveying belt. A high voltage, the polarity of which is reverse to that of the toner charging polarity, is impressed upon the transfer section using a constant current control so that the toner image is transferred. Superior transfer efficiencies and separation effects can be obtained in the transfer section of the transferring belt device.
If the transferring belt device is desirably used in a color image recording device in which toner images are superimposed on the photosensitive drum and transferred onto the recording sheet at one time, then in the image recording apparatus, it is necessary to provide a plurality of developing devices around the peripheral surface of the photoreceptor drum because toner images are superimposed on the photoreceptor, resulting in a large drum diameter. Therefore, sufficient sheet separation properties can not be obtained in the conventional electrostatic transfer separation method. Accordingly, more positive separation properties are necessary. In a method in which toner images are superimposed, a toner adhesion amount is large, and therefore, a large electric charge amount is necessary. Accordingly, a large transfer electric charge holding ability is necessary. Such a transferring belt device is superior in the foregoing two points.
However, in the image recording apparatus having a transferring belt device as shown in FIG. 1, when a corona discharger 36 is provided on the rear surface of the transferring belt 31 and corona discharging is conducted at a nip section N, at which a transferring/conveying belt 31 is stretched around a drive roller 32 and a driven roller 33 is contacted with a photosensitive drum 10, and transferring is conducted on a recording sheet P conveyed by the transferring/conveying belt 31, there are possibilities that sufficient transfer properties can not be obtained at the time of low humidity, and partial transfer due to carrier adhesion occurs.
Further, in the transferring device in which the rotational shafts of the driving roller 32, driven roller 33 or a tension roller 35 are not positioned accurately in parallel with each other in a frame body, the accuracy of an interval between the photosensitive drum 10 and the transferring/conveying belt is lowered at the nip section and thereby, inferior transfer occurs, or loads are unevenly provided on the transferring/conveying belt 31, so that the belt is skewed. However, when rollers 32, 33 and 35 are assembled in mass production, it is very difficult to provide them accurately in parallel with each other in the frame.
The inventors of the present invention have investigated causes of the foregoing unstable transfer properties, and found that the condition under which the transferring/conveying belt 31 is contacted with the photosensitive drum 10 is unstable. In order to solve the foregoing problems, as shown in FIG. 1, a spacing roller 39 is provided inside the transferring/conveying belt 31 at the upstream side of the nip section N at which the photosensitive drum 10 is contacted with the transferring belt 31 so that the transferring/conveying belt 31 is pressed to the photosensitive drum 10 by the spacing roller 39. As a result of the foregoing structure, the transferring/conveying belt 31 can keep the contacting condition with the photosensitive drum 10 within a wide range from the position of the spacing roller to the nip section, so that the foregoing inferior transfer does not occur.
However, several problems are not solved by the foregoing method. As will be explained later, the transfer/conveying belt is separated from the photosensitive drum 10 during the image formation, and the transferring belt 31 is contacted with the photosensitive drum 10 with pressure only when transferring. At that time, the pressing force works most strongly on the roller 39 which is newly provided, and therefore, it is necessary for the pressing force to be adjusted within an appropriate range. When the value of the pressing force is beyond the above-described range, an inferior transfer phenomenon, which is called transfer repelling, is caused, so that transfer properties are greatly lowered.
A skewed pressing force of the transferring/conveying belt 31 on the photosensitive drum 10 is caused when a stationary bearing section of the driving roller 32, which is the center of oscillation of a movable section of the transferring belt device, is not exactly in parallel with that of the spacing roller 39. In order to accurately provide parallel alignment, it is necessary to increase the machining accuracy, to add assembling and adjusting members, or to provide complicated adjusting processes. Further, according to the environmental conditions of temperature and humidity, the transferring/conveying belt expands and contracts, and therefore, uniform transfer properties can not be obtained.
Further, the present invention relates to the control of operations of a cleaning device by which an image carrier for image formation in a printer in the image forming apparatus or a copying apparatus is cleaned.
In conventional image formation, the surface of an image forming body which is formed by the photosensitive drum is charged; latent images are written on the surface; a plurality of color toner developers are developed a plurality of times (four times); finally, the developing surface is exposed; a color toner image is transferred onto the recording sheet by a belt type transfer means; and after that, the surface of the image forming body is cleaned by the cleaning blade.
In the conventional printer or copying apparatus as described above, the surface of a drum-shaped image forming body is cleaned by a blade type cleaning member after transfer has been completed. In this case, no toner remains on the image forming surface which has been cleaned by the blade type cleaning member, so that an excellent cleaning condition can be provided. However, a slight amount of color toner remains on an end of the cleaning blade. Especially, in a full color image forming apparatus in which it is necessary for the cleaning blade to be contacted with and released from the image forming surface, a ridge of toner remains on the image forming surface when the cleaning blade is removed from the image forming surface.
When cleaning is conducted only on the image forming surface on which the color toner image is formed, and from which the image is transferred, a release position of the cleaning blade is changed a little, and there occur many color toner remaining portions on the image forming surface. These remaining color toners enter into the developing device, adhere to the transferring belt, and adhere to the rear side of the recording sheet and stain it, which are problems.